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Old Sep 28, 2011, 02:52 AM
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Letchworth, Great Britain (UK)
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Question
Can I put too much power into a prop?

Many years ago I built a DC3 from plan. It's 63" wingspan and was designed for .10 glow engines. During the build I got into electric power, so converted it to electric with two AXI 2217/9D motors and 3S2P A123. I tried to fly it a while back, but hit a hedge when it didn't lift off within the length of our runway

My props then were APC E 7x5, about the largest stock diameter that could fit, and WebOCalc said it would fly like an intermediate trainer on that setup, at 20A per motor. I've now got 3-blade 7-1/2" Varioprops for it (just fit, with 1mm to spare) after the rebuild, set to about 4-1/2" pitch, and they're giving 29A static for 255W per motor, just over the motor's continuous rating of 27A, and well within its 60-second burst rating of 34A. So it seems good to go.

My puzzle is that, if I try to tweak WebOCalc to use the actual amps I'm seeing, it says the model won't fly because it can find no suitable prop, subject to the 7" size limit (it doesn't do 1/2" increments). Also, if I plug 3S LiPos into it, instead of A123s, it says the same.

So what I'm wondering is, is there a point at which a prop just thrashes around and consumes watts, rather than using the watts to produce thrust and speed? And, if so, am I likely to be over that limit with my setup?
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Old Sep 28, 2011, 03:50 AM
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Sounds more like a failing of the calculator to me. A program that refuses to believe in the equipment you're actually using is a bit pointless . Time for a different calculator perhaps ? I guess the calc is just falling foul of some fixed filters it insists on applying, possibly maximum prop revs. Have you tried it with an 8" prop ? It should believe in those and be willing to offer something but I don't know what you'd then interpret from it since that isn't the prop you're using any more than a 7" would be.

Do you have a tacho ? The revs you're getting might be interesting. It is sort of possible to get a prop absorbing too much power but that mainly means you're spinning it too fast and it's likely to destroy itself. With that relatively high Kv motor that might be a problem. What is the rpm limit for Varioprops ? You generally get a stalled (thrashing round to no real effect) prop with high pitch and high revs so I wouldn't think that would be much of a problem since you're running relatively low pitch.

Steve
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Old Sep 28, 2011, 05:08 AM
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Originally Posted by slipstick View Post
Sounds more like a failing of the calculator to me. A program that refuses to believe in the equipment you're actually using is a bit pointless . Time for a different calculator perhaps ? I guess the calc is just falling foul of some fixed filters it insists on applying, possibly maximum prop revs. Have you tried it with an 8" prop ? It should believe in those and be willing to offer something but I don't know what you'd then interpret from it since that isn't the prop you're using any more than a 7" would be. ...
This is the first time I've been limited significantly on prop size, and 7-1/2" is the absolute maximum that will fit on the model. In the past, my experience of WebOCalc has been good -- I put in a maximum prop size bigger than I really want, and it selects a range of suitable sizes which are very close to my real results when I run a static test. Anyway, I've re-run the calculation with 8" prop, and it's suggesting an 8x8 prop will be right for 29A

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Originally Posted by slipstick View Post
... Do you have a tacho ? The revs you're getting might be interesting. It is sort of possible to get a prop absorbing too much power but that mainly means you're spinning it too fast and it's likely to destroy itself. With that relatively high Kv motor that might be a problem. What is the rpm limit for Varioprops ? You generally get a stalled (thrashing round to no real effect) prop with high pitch and high revs so I wouldn't think that would be much of a problem since you're running relatively low pitch. ...
I don't have a tacho -- that's one for the Xmas list, I guess -- but my instinct about relatively low pitch (around 50% of diameter) is like you've said. Good point about the rpm limit though; the 7.5" blades are good for 14,500rpm, according to their packaging. My 255W per motor is at about 8.29v so, with kv=1880, that calculates out at 16,500rpm Maybe the tacho can't wait until Xmas.
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Old Sep 28, 2011, 07:41 AM
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Fortunately you won't be getting V x Kv, more like 80% of that I'd guess so you're probably o.k. But it might be close enough to be a bit worrying .

If it helps Motocalc reckons around 30A @ 13000rpm for that general setup with about 33oz thrust (per motor obviously) and 55 mph pitch speed. But I have no idea how well it models Varioprops which I believe can be a bit odd.

Good luck - Steve
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Old Sep 28, 2011, 07:45 AM
Jack
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"..This is the first time I've been limited significantly on prop size, and 7-1/2" is the absolute maximum that will fit on the model..."

If you can't add length maybe you can add blade width with one of the stronger slow fly props to increase the thrust. A GWS 7 x 6 would fit but would be limited to about 6.500 RPM by RPM limits (45,000/dia in inches).

The stronger slow flies like APC and Great Planes would get you about 2,000 more RPM with a 65,000/dia. in inches limit..

I like the Great Planes slow fly props better than the APC for flying traits, looks, weight, and looks. A couple of GP 8 x 6 props trimmed down to 7.5" might be a option I would try.

http://www.electrifly.com/miscproducts/gpmq6610.html

Jack
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Old Sep 28, 2011, 09:56 AM
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Moab, Utah, USA
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What I don't understand is why you even need a calc program to determine whether or not your airplane will fly. Since you already know how much power your motors are producing, all you need to do is make sure you have sufficient power loading and pitch speed to safely fly the airplane. With a DC-3 if the power loading is anything above 50 watts/lb and the pitch speed is anything above 2.5 times the stall speed, the airplane will fly just fine. IMHO these are calculations you should be doing with any airplane setup anyway.


1. To determine your power loading simply take your total of 510 watts and divide it by the airplane weight in pounds. For that type of airplane anything from 50 watts/lb to 70 watt/lb should be more than ample.

2. To determine your pitch speed multiply your RPM times your propeller pitch and divide by 1056. Sounds like your RPM will probably be about 70% of the kv times the input voltage, or about 12,000. Get a tachometer.

3. To determine your stall speed take the square root of the wing loading in ounces/sq ft and multiply that times 4.

4. To determine if you have sufficient pitch speed simply divide the pitch speed by the stall speed. Any result from 2.5 to 4 means your airplane has sufficient pitch speed to fly.


Larry
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Old Sep 28, 2011, 01:44 PM
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Originally Posted by slipstick View Post
Fortunately you won't be getting V x Kv, more like 80% of that I'd guess so you're probably o.k. ...
Yes, that's what I figured, thanks.

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Originally Posted by Lnagel
What I don't understand is why you even need a calc program to determine whether or not your airplane will fly. Since you already know how much power your motors are producing, all you need to do is make sure you have sufficient power loading and pitch speed to safely fly the airplane. With a DC-3 if the power loading is anything above 50 watts/lb and the pitch speed is anything above 2.5 times the stall speed, the airplane will fly just fine. IMHO these are calculations you should be doing with any airplane setup anyway.


1. To determine your power loading simply take your total of 510 watts and divide it by the airplane weight in pounds. For that type of airplane anything from 50 watts/lb to 70 watt/lb should be more than ample.

2. To determine your pitch speed multiply your RPM times your propeller pitch and divide by 1056. Sounds like your RPM will probably be about 70% of the kv times the input voltage, or about 12,000. Get a tachometer.

3. To determine your stall speed take the square root of the wing loading in ounces/sq ft and multiply that times 4.

4. To determine if you have sufficient pitch speed simply divide the pitch speed by the stall speed. Any result from 2.5 to 4 means your airplane has sufficient pitch speed to fly.
I guess all those calculations are exactly what WebOCalc and other programs do, aren't they? Anyway, my watts/lb is 102; my pitch speed is about 55; my stall speed is 20; so I have sufficient pitch speed to fly (pitch/stall = 2.75).

Thanks for those forumulae, Larry. Maybe I'll rely less on computerised calculators in the future. I already keep all my models' data in a spreadsheet, so it's just a matter of adding a few more columns to do these calculations. Just out of interest, what is the issue if my pitch speed/stall speed ratio is above 4?

Edit: Thanks for the info about slow-fly props, jackerbes. But they're all rated for much less rpm than my setup is pulling. One reason I've gone for 3-bladers, apart from more-scale, is to increase the blade area, as you suggest.
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Last edited by abenn; Sep 28, 2011 at 02:08 PM. Reason: More info.
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Old Sep 28, 2011, 03:12 PM
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Originally Posted by abenn View Post
Just out of interest, what is the issue if my pitch speed/stall speed ratio is above 4?
It's a matter of return verses investment. With speed drag increases exponentially and the power to overcome that drag even more so. On higher drag airplanes going above 3 or 4 on the pitch speed to stall speed ratio gets you only a relatively small increase in speed for a large increase in power. Essentially, it needlessly wastes power. Now on a low drag, high speed airplane you may want to go to a 5 or even 6 ratio.

Larry
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Old Sep 28, 2011, 03:18 PM
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Originally Posted by abenn View Post
Many years ago I built a DC3 from plan. It's 63" wingspan and was designed for .10 glow engines. During the build I got into electric power, so converted it to electric with two AXI 2217/9D motors and 3S2P A123. I tried to fly it a while back, but hit a hedge when it didn't lift off within the length of our runway

My props then were APC E 7x5, about the largest stock diameter that could fit, and WebOCalc said it would fly like an intermediate trainer on that setup, at 20A per motor. I've now got 3-blade 7-1/2" Varioprops for it (just fit, with 1mm to spare) after the rebuild, set to about 4-1/2" pitch, and they're giving 29A static for 255W per motor, just over the motor's continuous rating of 27A, and well within its 60-second burst rating of 34A. So it seems good to go.

My puzzle is that, if I try to tweak WebOCalc to use the actual amps I'm seeing, it says the model won't fly because it can find no suitable prop, subject to the 7" size limit (it doesn't do 1/2" increments). Also, if I plug 3S LiPos into it, instead of A123s, it says the same.

So what I'm wondering is, is there a point at which a prop just thrashes around and consumes watts, rather than using the watts to produce thrust and speed? And, if so, am I likely to be over that limit with my setup?
Whats the wing area and AUW of the plane?
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Old Sep 29, 2011, 02:11 AM
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460 square inches; 80 ounces.
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Old Sep 29, 2011, 06:18 AM
Jack
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That works out to 27 oz./sq. ft or 76.4 grams/dm2, that is a pretty darned high wing loading. You need lots of prop, lots of speed, and lots of room for takeoff...

Jack
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Old Sep 29, 2011, 07:16 AM
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Ok, let's take a look at what we have.

First off, WebOCalc is working fine and shows that with an APC 7x5E prop and a 3s A123 battery, the plane is going to struggle to fly. The twin motors will produce only about 1/2 the plane's weight in thrust... resulting in a maximum climb out angle of about 30 degrees. The pitch speed will be about 56 mph.... just barely enough, as it's only around twice the stall speed of the plane.

The plane will be running at about 28 watts per pound, and with a stall speed of nearly 25 miles per hour, I'm going to say that the plane won't leave the ground till it's traveling along the run way at over 30 mph. This is probably why you ran out of runway on the first go round. Also, the motors are not being used to their full potential, since they only draw around 16 or 17 amps static.

By switching to a decent 3s Li-Po, you'll get 70 ounces of thrust instead of 40. 70 degree climbouts instead of 30, and a pitch speed of 72 mph. The motor amp draw will be about 27 amps through each motor.

Takeoff speed will still be over 30 mph, since the wing loading is so high, but with the added thrust, it should be a lot easier to get it to take off speed.

WebOCalc is working just fine...

Chuck
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Old Sep 29, 2011, 07:25 AM
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WebOCalc is working just fine...
Interesting but how do you persuade it to work just fine with the 3-blade 7.5" Varioprops that the OP is actually using ?

Steve
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Old Sep 29, 2011, 07:29 AM
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Here ya go...
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Old Sep 29, 2011, 07:30 AM
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Interesting but how do you persuade it to work just fine with the 3-blade 7.5" Varioprops that the OP is actually using ?

Steve
No calculator program will work unless the prop constants are known. I highly doubt any of the prop guys have done the constants for his prop. If they are in fact known, I can add it to WebOCalc...
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